1. Field of the Invention:
This invention relates generally to electrooptical displays of the electrochromic type and, more particularly, to electrochromic display constructions having improved night visibility.
2. Description of the Prior Art:
It is generally known that electrooptical displays of the electrochromic type possess several advantages over the more widely used liquid crystal displays. For example, the electrochromic display generally exhibits much better legibility than a liquid crystal display, chiefly as a result of the former's higher contrast. Also, polarizers are not required in conjunction with electrochromic displays whereas they are with displays of the liquid crystal type. A problem common to both types of displays is to provide sufficient night viewability to the observer. In the area of liquid crystal displays, this problem has been solved by placing a conventional light source, such as an incandescent bulb, beta ray phosphor lamp, and the like behind the display for back lighting. Back lighting has proved effective since the components of the liquid crystal display can be made of materials transparent to the back light. However, at least with respect to some electrochromic display constructions, conventional back lighting by placing a light source behind the display is difficult or impossible to achieve because certain components of the display are opaque or nearly so to the back light. For example, the electrolytes used in electrochromic displays may be liquids, gels, pastes, or solids, many of which are not transparent. In addition, most electrochromic displays utilize a pigment material in the electrolyte gap to provide enhanced contrast to the upper electrochromic layers on the image-forming electrodes and to hide the lower electrochromic layer on the counter-electrode. For example, in an embodiment of the Leibowitz patent U.S. Pat. No. 3,944,333, a pigment layer is provided between a porous membrane containing a liquid electrolyte and the counter electrode. The counter electrode itself may be formed by metallizing a ceramic plate. Of course, the presence of either the pigment layer or metallized ceramic plate or both in the display prevents utilization of conventional back lighting to enhance night viewability.
As represented by U.S. Pat. No. 3,998,525, some prior art workers have attempted to improve night viewability of electrochromic displays by side or edge illumination of the display sandwich.
The use of radioactive light emitters to illuminate the periphery of a transparent watch dial is shown in U.S. Pat. No. 3,701,900. Radioactive microspheres in intimate contact with a phosphor are disclosed in U.S. Pat. No. 3,147,225. Discrete encapsulated electroluminescent materials are shown in the following patents: U.S. Pat. Nos. 2,881,344; 2,944,177; 2,976,446; 3,475,640; and 4,020,389. However, the phosphor materials are activated by electric fields rather than by radioactive decay particles.